Quoting Peter Wemm, "At great personal risk, touch the sendmail startup

Quoting Peter Wemm, "At great personal risk, touch the sendmail startup
again."

As an alternative to sendmail_enable=NONE, solve the boot time problem
for non-sendmail users completely by moving all of the sendmail startup
code from /etc/rc to /etc/rc.sendmail.  The source for that script will
be kept in src/etc/sendmail/rc.sendmail so make.conf's NO_SENDMAIL will
prevent it from being installed.  A new rc.conf variable,
mta_start_script specifies the script to run to start the user's
preferred MTA.  For backward compatibility, it will default to
/etc/rc.sendmail.  The specified script is called out of /etc/rc after
checking to make sure it exists.  A new rc.sendmail.8 man page has also
been added which now houses the sendmail_* variable descriptions
formerly in rc.conf.5.

Use /etc/rc.sendmail in /etc/mail/Makefile to reduce code duplication.

Reviewed by:	-current, -stable, obrien, peter, ru
MFC after:	1 week